Preview is accurate, but export / render is overexposed / too bright when there's a layer on top
A couple things occur to me:


Your Materials. I noticed your materials in the second screenshot. Why use an Emit value instead of cranking up the Intensity and checking Shading > Shadeless? This could bleed over and cause the brightening.
Text in the VSE. Is your text animated? Does it move or change? If it doesn't, you should really consider using Text strips instead of Scene strips. To add a Text strip, key Shift + A, then mouse over to Effect Strip > Text. In the VSE's Properties panel (key N to toggle open/close) make sure Edit Strip > Alpha Over is selected in the Blend: drop down menu. This will make sure that only the text is displaying. Lastly, go to VSE Properties panel > Effect Strip > Text: and enter your text.